1. Field of the Invention
This invention relates to a method for manufacturing a concrete structure. More particularly, it is concerned with a method for manufacturing a concrete structure whereby, in particular, existing concrete structural members such as columns or beams are reinforced in their shear strength.
2. Discussion of Background
Various existing building structures require reinforcement against earthquake, because they were constructed under old design standards and instructions, and thus are inferior in their aseismatic performance. Or, such aseismatic reinforcement is required for the sake of increasing the number of storeys of the building at the time of extending and/or remodeling the existing buildings.
As the representative method for the reinforcement against earthquake according to the conventional technique, it has been proposed to enclose the existing column members with steel plates or to envelope such existing column members with welded metal nets or reinforcing steel cages, in an attempt to improve toughness of the column members in the main, i.e. in an attempt not to reduce the loading capability and the energy absorbing capability, even when such structural elements are subjected to damage such as cracks, etc. to some extent.
This reinforcing method, however, unavoidably necessitates the welding work of the steel plates at the construction site, and in order to obtain the desired reinforcement, the welding work must be done by skilled welders.
Further, it is usual to pour mortar between the existing column members and the steel plates, welded metal nets, or reinforcing steel cages to attain transmission of stress between them. However, it has been difficult to fill such mortar compactly and densely between them.
Moreover, with the above described reinforcing method, it is common that slits are formed at the end portions of the reinforcing members such as steel plates to increase the shear strength alone of the existing column members, while retaining the bending strength of the reinforcing members to be the same as before the reinforcement. With such measures, however, it is inevitable that, at such slitted portions, the structural members exposed to the environment have poor water-tightness against rain. Consequently, a trouble of leakage of water is likely to occur.
Further, with the reinforcing method using steel plates, it is necessary to apply rust-preventing treatment to the steel plates, which adds to the maintenance cost.
Furthermore, the conventional way of applying the reinforcing member to the concrete structural member has another disadvantage such that when cracks are formed in the concrete structural member, stress tends to concentrate on the reinforcing member in the vicinity of locations where such cracks are formed in the concrete structural member, since both the concrete structural member and the reinforcing member are integrally connected by various adhesives. As a result, the reinforcing member is likely to break at a stage when the cracks are still small in size (width), whereby it is impossible to utilize the strength of the reinforcing member to the fullest extent.
A spirally reinforced column made of reinforced concrete material has so far been assembled in such a manner that a reinforcing steel wire is spirally wound from one end to the other end of the intended structural member. According to this method, the spiral reinforcing steel wire is fixed to the end portions of the structural member where the stress is large and the plastic deformation concentrates.
If, instead of the spiral reinforcing steel wire, a flexible reinforcing member such as a reinforcing fiber, is wound around the structural member in accordance with the above method, such plastic deformation concentrates at the end portions of the structural member as mentioned above, and fixing of the fiber on the structural member deteriorates with a consequent reduction in the stress occurred in the axial direction of the fiber. Since the fiber can hardly bear a stress in a direction other than the axial direction, the reinforcing effect with the fiber will then be extremely small.
On the other hand, it may be contemplated to extend the fiber in the direction of its winding so that it is fixed at another storey of the building instead of at the end portions of the structural member. According to this method, however, the reinforcing fiber inevitably passes through a junction of the column and the beam, where a large stress concentration occurs, and it becomes difficult to maintain the structural performance properly at this junction of the column and the beam. In addition, it is usual in the building construction that the work itemization and the process control of the construction work are planned for each and every storey, so that, when the work in one storey should extend to another storey (such as fixing of the fiber reinforcing material at another storey), such would bring about restriction to the management and control in the construction work.
The present inventors have previously proposed a method of reinforcing a column, in which a long fiber strand having high mechanical strength is spirally wound on the column member (Japanese Patent Application Nos. 273357/1984 and No. 109267/1985). This method lets the high strength long fiber strand as the reinforcing member have a function as a spiral hoop for the reinforced concrete column, from which both effects of increase in the strength of the column and improvement in its toughness can be expected. It has, however, been found that, when cracks are created in the concrete of the column member which has been reinforced by this method, and the strand is broken as a result of the concentration of stress on the fiber strand situated in the vicinity of the cracks, the binding force of this strand becomes abruptly lowered, and the reinforcing effect will be considerably reduced.
Also, in the course of further researches and experiments on the above described method of reinforcement, the present inventors have found also that the cracks start in the column member from its upper end where it is joined with the beam, or from its lower end where it is joined with the floor; that, when the long fiber strand for reinforcement wound around such portion of the column member is broken at the initial stage, the reinforcing effect of the fiber strand as a whole becomes considerably reduced; and that the mortar which covers the outer surface of the column member tends to scale off at the initial stage of the earthquake, and, in this case, if the pitch for winding the fiber strand is large, it is difficult to effectively bind the coarse aggregate beneath the mortar, whereby the coarse aggregate falls off together with the mortar, and no adequate reinforcing effect can be obtained.
It has also been found out that, since the fiber strand is wound around the structural member (column) in a spiral form, it is not possible to obtain sufficient binding force of the fiber strand at both upper and lower ends of the column, where the winding direction of the reinforcing material is reversed, and that, at both the beginning and the end of winding of the fiber strand, it is impossible to transmit a high tension to the fiber strand.